Maybe for GNU project contributors, but look at some of the most currently popular FOSS projects. They all came from companies that rely heavily on proprietary software for their core business. They're certainly not contributing to FOSS because they're ideologues who believe all software should be FOSS.
Kubernetes (Google), Angular (Google), TensorFlow (Google), VSCode (Microsoft), TypeScript (Microsoft), React (Facebook), PyTorch (Facebook), just to name a few.
Kubernetes (Google), Angular (Google), TensorFlow (Google), VSCode (Microsoft), TypeScript (Microsoft), React (Facebook), PyTorch (Facebook), just to name a few.